Traditionally, an underground hydrocarbon reservoir was developed by drilling a vertical well into the formation. If it appeared that the well had located commercial quantities of oil or gas, the well would be completed. Completion usually involves a process known as "setting pipe." "Setting pipe" involves lowering a continuous string of production casing pipe into the hole and cementing it in place. A perforating gun is then lowered into the casing to the depth of the potential petroleum bearing rock. The casing, cement and several inches of rock would then be perforated by explosives in the gun, allowing petroleum in the formation to drain into the wellbore. Sometimes it is necessary to stimulate the well by fracturing the rock hydraulically or through acid treatments.
If the natural pressure within the rocks is high, oil will flow to the surface. If the pressure is low, pumping equipment will be installed to lift the oil to the surface. After some period of time, ranging from several months to many years, the natural or primary pressure in the reservoir rocks may drop to such a level that hydrocarbons will no longer flow into the wellbore at economically producible rates. At that time, secondary recovery techniques may be employed such as water flooding or carbon dioxide flooding of the formation.
More recently, a new technique has been developed to increase production from reservoirs. A wellbore is drilled to match the orientation of the hydrocarbon bearing formation. As these formations are usually horizontal such wells are known as "horizontal wells" or "drain holes." A horizontal well, therefore, is a well which is not vertical and which has been deviated from vertical to increase its contact with hydrocarbon bearing formation. A horizontal well is initiated as a vertical well near the surface. However, as the wellbore's depth increases, it is generally deviated from vertical until its orientation is substantially horizontal thus matching the orientation of the hydrocarbon formation. Although they are more costly and difficult to drill, horizontal wells offer several advantages over vertical wells. One advantage is the increase in direct contact between the horizontal wellbore and the hydrocarbon producing zone or pay zone. The perforated interval for a vertical well is limited to the width of the pay zone. But for a horizontal well, the perforated interval could be many times that of a vertical wellbore. Furthermore, this increase in length allows for an increased number of potential fracture locations. For example, a vertical well might only be fractured in three locations, while a horizontal well could be fractured at, for example, up to fifteen locations.
Horizontal wells, however, have several disadvantages. A first disadvantage involves the ability to lift fluid out of a horizontal wellbore. Producing a low pressure reservoir through a horizontal or near horizontally drilled wellbore with conventional artificial lift equipment is either impossible or very expensive. Lift equipment, such as "roller rods", can only produce from the highest or a higher point in the wellbore. Even then, some reservoir pressure is required to raise the hydrocarbons to that point.
A second disadvantage involves fracture stimulation along a horizontal wellbore. Hydraulic fracture stimulation of a conventionally drilled drain hole can only be applied along the single wellbore from a single surface opening. This requires expensive mechanical isolation for creating and fracturing of multiple fractures over the length of the drain hole. Also, fracturing rates are limited to the capacity of one wellbore and are often inadequate for stimulation of the large amount of reservoir requiring multiple fracture stimulation. Furthermore, fracture treatments often "screen out" in the long horizontal drain hole due to the large surface area encountered and inadequate fracturing rates and pressure at the formation fracture point. Besides the difficulty of supplying adequate stimulation fracturing rates and pressure, horizontal wells create difficulties in controlling fracturing rates and pressures. For example, when drain holes are completed with uncemented slotted liners, multiple stage fracture treatments using diverting agents are required to open multiple fractures. Diverting agents and volumes are difficult to calculate and control over these long sections where that control is critical for multiple fracture initiation and access to the reservoir is only from one end of the drain hole.
A number of patents have issued on methods of producing hydrocarbons incorporating horizontal wellbores. For example, U.S. Pat. No. 4,682,652 to Huang et al. discloses a method of producing hydrocarbons through successively perforated intervals of a horizontal well between two vertical wells. The method requires a horizontal well to be drilled under the vertical wells. This horizontal well is then perforated along its length. The first vertical well is injected with thermal fluid. Hydrocarbons are produced first through the perforations closest to said vertical well, and later through successive perforations even farther from the first vertical well. Ultimately, hydrocarbons are produced by the second vertical well. While this method coordinates the use of horizontal wells with existing vertical well patterns, the vertical wells do not contribute to the fracturing of the formation surrounding the horizontal well.
U.S. Pat. No. 4,532,986 to Mims et al. discloses a method of completing a well involving the intersection of a horizontal well with a vertical well. The completion includes a well liner which lies in a generally horizontal disposition within a hydrocarbon holding substrate to define the primary well. A secondary well which extends to the surface intersects the primary well. A stream of hot stimulating fluid is injected into the primary well from the secondary well. A flow diverter is positioned in the primary well to urge the stimulating fluid into the substrate at desired locations. The fluid creates a heated path along which viscous oil may flow. Mims et al. does not disclose a method of fracturing the formation surrounding the primary well. Nor does Mims et al. disclose a method of stimulating a well simultaneously from both well openings.
U.S. Pat. No. 4,390,067 to Willman discloses a method for treating a field containing viscous oil for subsequent production. The method involves drilling a horizontal well within the oil-bearing stratum and heating the oil in the vicinity of the horizontal well to produce a hot liquid corridor. The horizontal and vertical wells may be connected in various configurations to effectively displace a high percentage of oil in a particular field.
In sum, many older producing fields, where reservoir pressure has been depleted, cannot economically support the drilling of many additional vertical wells between the existing wells in order to produce the remaining hydrocarbons. Moreover, horizontally drilled drain holes are difficult to produce where reservoir pressure is low or depleted and will not lift fluid to a point high enough to produce economically. A need exists for a method of producing low pressure reservoirs with horizontal wells which overcomes the difficulties encountered in production and stimulation of the horizontal well.